David Cameron and Ed Miliband have jointly urged Scotland to reject calls for independence if a referendum is held on the subject.

The prime minister told MPs he believed "passionately" in the Union, while the Labour leader said its break-up would be damaging to the UK's economy.

Scotland's First Minister Alex Salmond has said he will hold a referendum on independence in autumn 2014.

But the coalition says any a vote would be unlawful without their approval.

The Conservatives, Liberal Democrats and Labour are opposed to Scottish independence and the leaders of all three parties have promised to fight against the cause championed by Mr Salmond's Scottish National Party (SNP).

The Scottish government insists it can go ahead with a referendum without the need for a law to be passed at Westminster - but the coalition disputes this.

At Prime Minister's Questions, Mr Cameron called for cross-border talks with Scottish ministers in an effort to end the constitutional dispute.

'Never-endum'

He told MPs: "I passionately believe in the future of our United Kingdom. I passionately believe we are stronger together rather than breaking apart."

He added: "But we have to respect the fact that Scotland voted for a separatist party at those Scottish parliamentary elections, so the first thing it is right to do is to make clear the legal position about the referendum."

"Too many in the SNP are happy to talk about the process; they don't want to talk about the substance.

"It's not a referendum they want; it's a never-endum. Let's have the debate and let's keep our country together."

Mr Miliband agreed, telling the Commons: "We are stronger together and weaker apart."

He called for "immediate cross-party talks" in Scotland about the timing of the vote and the nature of the "single question" which would be asked in a referendum.

Mandate

Scotland's First Minister Mr Salmond said he wanted a referendum "which is built and made in Scotland, produced by the Scottish Parliament and offered to the Scottish people".

He said the SNP - which won a majority at Holyrood last year - would "never rule out talking to anyone", but insisted the party already had the mandate to act.

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Most people would think the proposals so far were "fair enough" and would wonder why "the PM would want to trample over Scotland in size 10 boots", he told the BBC.

He accused Mr Cameron of "wanting to set the ground rules for our referendum despite the fact it has got no mandate whatsoever for doing so".

The SNP says it is "entirely confident" of its plans to bring in a Referendum Bill setting up a vote for autumn 2014.

But the Westminster government argues that any referendum should take place "sooner rather than later" and says the power to hold a referendum is "reserved" to Westminster under devolution laws passed in 1998.

If Westminster and Holyrood fail to resolve the legal issue it could end up being ruled over by the Supreme Court.

Meanwhile, Mr Salmond has dismissed suggestions that an independent Scotland might have to give up the pound and join the euro as "the stuff of nonsense".

"We would stay with sterling until the people of Scotland decided otherwise in a referendum," he told BBC Two's Newsnight.